The initiation and termination of the cell cycle may be under translational control. The maturation of transfer ribonucleic acid, then, is of particular interest. The efficiency with which tRNA interacts with the enzymes of the protein synthetic apparatus may regulate the rate of protein synthesis and thereby the rate of cell division. Work to support this notion will involve the structure and metabolism of tRNA from regenerating rat liver and Novikoff ascites tumor cells. The results are particularly appropriate in the understanding of events leading to a breakdown of the regulation of the cell cycle in neoplastic cells. Specifically, experiments designed to answer the following questions will be undertaken: 1. What are the sequence differences of tRNA phe/II isolated from normal and regenerating liver? What are the differences in unusual nucleoside content of the purified isoacceptors derived from the two sources? 2. Can one differentially repress tRNA methylation or synthesis in Novikoff ascites cells with appropriate inhibitors? Is there a casual relationship between limitation of protein synthesis in tumor cells and inhibition of tRNA modifying enzymes? 3. Are there two classes modifying enzymes, those obligate for function and those facilitative? If so, is tRNA efficiency regulated by the former or the latter? 4. Are isolated, purified tRNA met/f or tRNA phe/II from regenerating liver more efficient in amino acylation, ribosome binding, or chain elongation assays than the corresponding tRNA's isolated from normal liver.